Braking energy recuperation performance of input coupled power-split hydraulic hybrid powertrain

Abstract: The fuel economy of the heavy-duty vehicles in China is relatively lower than that of advanced technologies in the world, and the fuel consumption of heavy duty commercial vehicles are very high, so energy-saving and advanced powertrain technologies for fleet users are urgently required, especially for the vehicles operating under city driving condition with heavy traffic jams. The power hybrid technologies are the best solution to improve the efficiency of heavy-duty commercial vehicles. Due to the benefit of high power density, hydraulic hybrid powertrains have more advantages than hybrid electric powertrains, especially under an urban duty cycle. Among all the configurations of hydraulic hybrid, the compound configuration has great energy saving potential. Targeting to improve the fuel economy of commercial vehicles with input couple hydro-mechanical transmissions operating under condition of frequent stop-and-go, the performance of braking energy recovery was examined. Many researches on regenerative braking performance of output couple hydro-mechanical transmissions have been conducted, but less has been reported on input coupled types. However, the input coupled hydro-mechanical transmissions were applied more widely than the output coupled ones. In the paper, the regenerative braking condition was investigated. When , the hybrid powertrain operating status is the power cycle condtion, and when , the hybrid powertrain operating status is the power split condtion.The power flow under different braking scenarios was analyzed, including the power-split condition braking and the power cycle condition braking. Based on the different braking scenarios, the equations of speed, flow, and torque balance were proposed to get operating region for recovering braking energy. Under different modes, the hydraulic unit 1 and 2 adjusting strategy was determined. Under the power cycle condition, the hydraulic unit 1 and hydraulic unit 2 must be adjusted by the rules of and. Under the power cycle condition, if the condition is met, the flow field for braking energy recovery is very small, and the fluid flow is low. Under the power split condition, the hydraulic unit 1 and hydraulic units 2 must be adjusted by the rules of and. If braking under the power split condition, the rotary speed of hydraulic unit 1 and hydraulic unit 2 decreased synchronously. In the first stage of braking, hydraulic units 2 acted as a pump. If the torque equation was met, the vehicle speed down to some value, the hydraulic unit 2 will rotate reversely and act as a hydraulic motor. To recovery the braking energy, the hydraulic unit 1 must operate in the reverse direction, acting as a hydraulic motor. To verify the analysis, the bench test for an input coupled hydraulic hybrid transmission was set up, and the braking energy recovery testing was done. The results show that for an input coupled type hydraulic hybrid powertrain, the braking energy recuperation efficiency within a low speed region is fairly low, and within a medium-high speed region, the hydraulic units adjusting areas were two separate parts with an invalid middle zone. From the view of system efficiency, the regenerative braking potential at the high-speed region is higher than at the low speed region. For hybrid system design, the multi-range transmission configurations were strongly recommended.